"Single people," a divorced man in his 30s once told Francine Klagsbrun, "think all long-married people are cowards." Klagsbrun, a Brooklyn-born writer with a dozen books and 30 years of marriage behind her, was irritated. "I've become really tired of the negativism, the knocking of marriage that has gone on for the last 10 or 15 years," she says. "It's time to present the positives." To find out just what it is that makes marriages last, she interviewed more than 100 couples whose marriages had endured 15 or more years of domestic travail. Klagsbrun's findings are reported in Married People: Staying Together in the Age of Divorce (Bantam, $16.95). As a wife (of Dr. Samuel Klagsbrun) and mother (of Sarah, 16), the author regards her book as fulfilling a mission. "I'm a feminist," she says, "and I think it's a good thing to speak out for the many people in the feminist camp who believe in marriage." She discussed her conclusions with reporter Lee Powell.

The marriage rate has increased in the 1980s, and more divorced people are remarrying. Why?

There's a rootlessness in our society today, and people want something to hold onto. We've been so open and had so many options, but now a lot of divorced people are finding loneliness and pain. They are beginning to look at the way their parents and grandparents did it and to think, "They didn't seem to be so unhappy."

Has the divorce rate declined?

It has, a little. Nobody is certain why. Perhaps because it's more expensive to divorce than to marry—for the man who has to pay alimony and for the woman who has to live on her own. That's one speculation. I've come to think it's because people are finding out it's not so easy to make a new life.

You interviewed people who married at least 15 years ago. What problems do people who marry today have that those couples didn't?

I think the problems now have a lot to do with dual occupations. The roles in marriage aren't clearly divided any longer. Most women intend to work, or at least have the option. So there's a fatigue factor at the end of the day, with both partners exhausted. And there's competition between two working people, especially if the wife does better than her husband. Society hasn't changed enough to make that acceptable. And there are many more opportunities for married women to meet other attractive men that they work with and have a lot in common with.

Does the increase in premarital sex mean that married women are less tempted by curiosity to become involved in an extramarital affair?

I hope it does. Sex therapists tell me some women miss the excitement of going from one affair to another, but I think it can help if a woman isn't wondering about her husband, "How do I know he's a good lover if I never had any other?"

Do young people marrying today have a shaky concept of "forever"? About the world as well as about marriage?

Young people are very conscious of the nuclear threat, but I don't sense it's what's affecting commitment to marriage. In the last 20 years there's been a lot of affluence, and young people have gotten pretty much what they wanted. They aren't used to dealing with frustration, and they don't have a tolerance for it, an inclination to stick with something.

What problems are the same for young people as they were for your couples?

The basic problems of closeness, of talking to each other, of caring—those are all the same.

What are some elements you found in every long-lasting marriage that was based on a good, strong relationship?

Change and ability to tolerate change—that's the heart of any marriage, because you will be many different people during the course of it. That's part of the excitement—seeing the changes in each other and being able to go along with them. You don't want to get stuck and say, "This is the man I married and I don't want him any different." You have to have flexibility. A job promotion can change your expectations quickly. When you have a baby, things change fast. And if one or the other partner is saying, "I don't want this," it can cause havoc.

But aren't married people sometimes frustrated by the failure of their partners to change?

There are some things about a person that aren't going to change, and it takes time to learn that. Some are little things that drive you crazy. For example, my husband won't empty the damned dishwasher, and he always leaves the bath mat on the floor. I used to scream and yell about it. But at some point you say, "All right, I'll do those things because there's no point in fighting about them. There are other things he does that are wonderful." And you may also have to live with more serious conflicts that don't get resolved.

For example?

Suppose your husband doesn't realize his career expectations, and it's a real disappointment to you. You can leave, or you can fight all the time or you can say: "This is who he is and we still have other satisfactions and we'll live with this." If you want to be negative, you could say that means settling for less. I call it maturity—recognizing realities. It's much like we are with our children. At first you think your child is perfect and can do everything. As the years go by you see what his abilities and limitations really are, and if you're a halfway decent parent, you make accommodations. One element that keeps a marriage going is accepting a person for what he or she is, for what will nor change.

Is there anything that should never be tolerated?

Yes. You don't accept someone constantly playing around outside the marriage. And you don't accept physical or constant verbal abuse to yourself or your child. You don't accept what is destructive or mean-spirited. You don't accept total blandness and boredom and emptiness. I'm not against divorce if people are miserable in a rotten marriage.

Does the assumption by two people that their marriage is going to be permanent play an important part in making it so?

Yes, if it represents a sense of commitment and an ongoing philosophy of marriage. One woman told me recently, "My husband and I made a deal that we're not going to break up." So when they fight they know they've got to work things out. And they do.

You talk about building trust. Don't people have to have that at the beginning of a marriage?

Yes, but it takes time to develop the really deep trust that means you feel absolutely safe with each other. It comes from experience, learning that this person is there for you when you're down and doesn't turn your weakest moments against you. At first in a marriage there's a tendency for people to hurt each other. I mean, you want to win those fights, and that's how you do it.

You say good marriages require a balance of dependencies. What does that mean?

In a good marriage a balance evolves from the shifting of dependency from one partner to the other. Neither partner should be totally dependent—each has to have a sense of self. But in the beginning there is often one partner who loves less and has more power emotionally. To get to a level of real trust, that power has to shift back and forth, and the shifting balance creates a kind of equality.

Why is a shared history so important in keeping marriages going?

Every marriage has a story, and people in long and good marriages cherish it. They don't want to give up what's between them. The good things ("Remember what we did with the kids?") and even the bad ("God, we've been through that") are all part of that, and looking back on it can help people work out whatever they're going through.

You say successful couples also need a little luck. How can they make sure they have it?

Some luck is a matter of chance. But I think sometimes you make your own luck by grabbing hold of what you have and making the best of it. Envy just eats you up alive. I was awed by people who had been through so many difficulties and yet were still together. They were able to say, "We're really lucky"—that their child hadn't suffered when he died, that they got through their problems with alcoholism. Terrible things. But they truly considered themselves fortunate.

What do you consider the greatest threat to your own daughter's chances of making a succcessful marriage?

Maybe being attracted to a man who is not supportive and kind. Women are often attracted to clever men who put them down. Also, I worry about her sense of priorities. We're pushing our kids, encouraging them to have careers—and what you achieve is very important. But what will you turn out to be as a human being? So that's what I worry about. Have I done enough to make her know the value of relationships?

If you had to use one word to describe what it takes to have a good marriage, what would it be?

Commitment. I wish there were another word, because it's overused. But that's it. Commitment to all the elements that make a lasting relationship. Commitment to each other. Commitment to the marriage.